37,972 research outputs found
The Formation of a Magnetic Channel by the Emergence of Current-carrying Magnetic Fields
A magnetic channel - a series of polarity reversals separating elongated flux
threads with opposite polarities - may be a manifestation of a highly
non-potential magnetic configuration in active regions. To understand its
formation we have carried out a detailed analysis of the magnetic channel in AR
10930 using data taken by the Solar Optical Telescope/Hinode. As a result, we
found upflows (-0.5 to -1.0 km/s) and downflows (+1.5 to +2.0 km/s) inside and
at both tips of the thread respectively, and a pair of strong vertical currents
of opposite polarity along the channel. Moreover, our analysis of the nonlinear
force-free fields constructed from the photospheric magnetic field indicates
that the current density in the lower corona may have gradually increased as a
result of the continuous emergence of the highly sheared flux along the
channel. With these results, we suggest that the magnetic channel originates
from the emergence of the twisted flux tube that has formed below the surface
before the emergence.Comment: 11 figures, accepted for Astrophysical journa
Thermodynamics of Four-Flavour QCD with Improved Staggered Fermions
We have calculated the pressure and energy density in four-flavour QCD using
improved fermion and gauge actions. We observe a strong reduction of finite
cut-off effects in the high temperature regime, similar to what has been noted
before for the SU(3) gauge theory. Calculations have been performed on
and 16^4 lattices for two values of the quark mass,
and 0.1. A calculation of the string tension at zero temperature yields a
critical temperature for the smaller
quark mass value.Comment: 12 pages, LaTeX2e File, 11 encapsulated postscript file
Recurrent Coronal Jets Induced by Repetitively Accumulated Electric Currents
Three extreme-ultraviolet (EUV) jets recurred in about one hour on 2010
September 17 in the following magnetic polarity of active region 11106. The EUV
jets were observed by the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory (SDO). The Helioseismic and Magnetic Imager (HMI) on board
SDO measured the vector magnetic field, from which we derive the magnetic flux
evolution, the photospheric velocity field, and the vertical electric current
evolution. The magnetic configuration before the jets is derived by the
nonlinear force-free field (NLFFF) extrapolation.
We derive that the jets are above a pair of parasitic magnetic bipoles which
are continuously driven by photospheric diverging flows. The interaction drove
the build up of electric currents that we indeed observed as elongated patterns
at the photospheric level. For the first time, the high temporal cadence of HMI
allows to follow the evolution of such small currents. In the jet region, we
found that the integrated absolute current peaks repetitively in phase with the
171 A flux evolution. The current build up and its decay are both fast, about
10 minutes each, and the current maximum precedes the 171 A by also about 10
minutes. Then, HMI temporal cadence is marginally fast enough to detect such
changes.
The photospheric current pattern of the jets is found associated to the
quasi-separatrix layers deduced from the magnetic extrapolation. From previous
theoretical results, the observed diverging flows are expected to build
continuously such currents. We conclude that magnetic reconnection occurs
periodically, in the current layer created between the emerging bipoles and the
large scale active region field. It induced the observed recurrent coronal jets
and the decrease of the vertical electric current magnitude.Comment: 10 pages, 7 figures, accepted for publication in A&
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